Magnetic recording medium

ABSTRACT

The magnetic recording medium includes a magnetic layer containing ferromagnetic powder and a binder, in which the ferromagnetic powder is ferromagnetic hexagonal ferrite powder, the magnetic layer contains an abrasive, an intensity ratio of a peak intensity of a diffraction peak of (110) plane of a crystal structure of the hexagonal ferrite, determined by performing X-ray diffraction analysis on the magnetic layer by using an In-Plane method, to a peak intensity of a diffraction peak of (114) plane of the crystal structure is equal to or higher than 0.5 and equal to or lower than 4.0, a squareness ratio of the magnetic recording medium in a vertical direction is equal to or higher than 0.65 and equal to or lower than 1.00, and a coefficient of friction measured in a base material portion within a surface of the magnetic layer is equal to or lower than 0.30.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2017-140017 filed on Jul. 19, 2017. The aboveapplication is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic recording medium.

2. Description of the Related Art

Generally, either or both of the recording of information on a magneticrecording medium and the reproduction of information performed bycausing a magnetic head (hereinafter, simply described as “head” aswell) to contact and slide on a surface of the magnetic recording medium(a surface of a magnetic layer).

In order to continuously or intermittently repeat the reproduction ofthe information recorded on the magnetic recording medium, the head iscaused to repeatedly slide on the surface of the magnetic layer(repeated sliding). For improving the reliability of the magneticrecording medium as a recording medium for data storage, it is desirableto inhibit the deterioration of electromagnetic conversioncharacteristics during the repeated reproduction. This is because amagnetic recording medium in which the electromagnetic conversioncharacteristics thereof hardly deteriorate during the repeated slidingcan keep exhibiting excellent electromagnetic conversion characteristicseven though the reproduction is continuously or intermittently repeated.

Examples of causes of the deterioration of electromagnetic conversioncharacteristics during the repeated sliding include the occurrence of aphenomenon (referred to as “spacing loss”) in which a distance betweenthe surface of the magnetic layer and the head increases. Examples ofcauses of the spacing loss include a phenomenon in which whilereproduction is being repeated and the head is continuously sliding onthe surface of the magnetic layer, foreign substances derived from themagnetic recording medium are attached to the head. In the related art,as a countermeasure for the head attachment occurring as above, anabrasive has been added to the magnetic layer such that the surface ofthe magnetic layer performs a function of removing the head attachment(for example, see JP2005-243162A).

SUMMARY OF THE INVENTION

It is preferable to add an abrasive to the magnetic layer, because thenit is possible to inhibit the deterioration of the electromagneticconversion characteristics resulting from the spacing loss that occursdue to the head attachment. Incidentally, in a case where thedeterioration of the electromagnetic conversion characteristics can besuppressed to a level that is higher than the level achieved by theaddition of an abrasive to the magnetic layer as in the related art, itis possible to further improve the reliability of the magnetic recordingmedium as a recording medium for data storage.

The present invention is based on the above circumstances, and an aspectof the present invention provides for a magnetic recording medium inwhich the electromagnetic conversion characteristics thereof hardlydeteriorate even though a head repeatedly slides on a surface of amagnetic layer.

An aspect of the present invention is a magnetic recording mediumcomprising a non-magnetic support and a magnetic layer which is providedon the support and contains ferromagnetic powder and a binder, in whichthe ferromagnetic powder is ferromagnetic hexagonal ferrite powder, themagnetic layer contains an abrasive, an intensity ratio (Int (110)/Int(114)) (hereinafter, described as “XRD (X-ray diffraction) intensityratio” as well) of a peak intensity Int (110) of a diffraction peak of(110) plane of a crystal structure of the hexagonal ferrite, determinedby performing X-ray diffraction analysis on the magnetic layer by usingan In-Plane method, to a peak intensity Int (114) of a diffraction peakof (114) plane of the crystal structure is equal to or higher than 0.5and equal to or lower than 4.0, a squareness ratio of the magneticrecording medium in a vertical direction is equal to or higher than 0.65and equal to or lower than 1.00, and a coefficient of friction measuredin a base material portion within a surface of the magnetic layer isequal to or lower than 0.30.

In one aspect, the squareness ratio in a vertical direction may be equalto or higher than 0.65 and equal to or lower than 0.90.

In one aspect, the coefficient of friction measured in a base materialportion within a surface of the magnetic layer may be equal to or higherthan 0.15 and equal to or lower than 0.30.

In one aspect, the magnetic recording medium may further comprise anon-magnetic layer containing non-magnetic powder and a binder betweenthe non-magnetic support and the magnetic layer.

In one aspect, the magnetic recording medium may further comprise a backcoating layer containing non-magnetic powder and a binder on a surface,which is opposite to a surface provided with the magnetic layer, of thenon-magnetic support.

In one aspect, the magnetic recording medium may be a magnetic tape.

According to an aspect of the present invention, it is possible toprovide a magnetic recording medium in which the electromagneticconversion characteristics thereof hardly deteriorate even though a headis caused to repeatedly slide on a surface of a magnetic layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An aspect of the present invention relates to magnetic recording mediumincluding a non-magnetic support and a magnetic layer which is providedon the support and contains ferromagnetic powder and a binder, in whichthe ferromagnetic powder is ferromagnetic hexagonal ferrite powder, themagnetic layer contains an abrasive, an intensity ratio (Int (110)/Int(114)) of a peak intensity Int (110) of a diffraction peak of (110)plane of a crystal structure of the hexagonal ferrite, determined byperforming X-ray diffraction analysis on the magnetic layer by using anIn-Plane method, to a peak intensity Int (114) of a diffraction peak of(114) plane of the crystal structure is equal to or higher than 0.5 andequal to or lower than 4.0, a squareness ratio of the magnetic recordingmedium in a vertical direction is equal to or higher than 0.65 and equalto or lower than 1.00, and a coefficient of friction measured in a basematerial portion within a surface of the magnetic layer is equal to orlower than 0.30.

In the present invention and the present specification, “surface of themagnetic layer” refers to a surface of the magnetic recording medium onthe magnetic layer side. Furthermore, in the present invention and thepresent specification, “ferromagnetic hexagonal ferrite powder” refersto an aggregate of a plurality of ferromagnetic hexagonal ferriteparticles. The ferromagnetic hexagonal ferrite particles areferromagnetic particles having a hexagonal ferrite crystal structure.Hereinafter, the particles constituting the ferromagnetic hexagonalferrite powder (ferromagnetic hexagonal ferrite particles) will bedescribed as “hexagonal ferrite particles” or simply as “particles” aswell. “Aggregate” is not limited to an aspect in which the particlesconstituting the aggregate directly contact each other, and alsoincludes an aspect in which a binder, an additive, or the like isinterposed between the particles. The same points as described abovewill also be applied to various powders such as non-magnetic powder inthe present invention and the present specification.

In the present invention and the present specification, unless otherwisespecified, the description relating to a direction and an angle (forexample, “vertical”, “orthogonal”, or “parallel”) includes a margin oferror accepted in the technical field to which the present inventionbelongs. For example, the aforementioned margin of error means a rangeless than a precise angle ±10°. The margin of error is preferably withina precise angle ±5°, and more preferably within a precise angle ±3°.

Regarding the aforementioned magnetic recording medium, the inventor ofthe present invention made assumptions as below.

The magnetic layer of the magnetic recording medium contains anabrasive. The addition of the abrasive to the magnetic layer enables thesurface of the magnetic layer to perform a function of removing the headattachment. However, it is considered that in a case where the abrasivepresent on the surface of the magnetic layer and/or in the vicinity ofthe surface of the magnetic layer fails to appropriately permeate theinside of the magnetic layer by the force applied thereto from the headwhen the head is sliding on the surface of the magnetic layer, the headwill be scraped by contacting the abrasive protruding from the surfaceof the magnetic layer (head scraping). It is considered that in a casewhere the head scraping that occurs as above can be inhibited, it ispossible to further inhibit the deterioration of the electromagneticconversion characteristics caused by the spacing loss.

Regarding the aforementioned point, the inventor of the presentinvention assumes that in the ferromagnetic hexagonal ferrite powdercontained in the magnetic layer include particles (hereinafter, referredto as “former particles”) which exert an influence on the degree ofpermeation of the abrasive by supporting the abrasive pushed into theinside of the magnetic layer and particles (hereinafter, referred to as“latter particles”) which are considered not to exert such an influenceor to exert such an influence to a small extent. It is considered thatthe latter particles are fine particles resulting from partial chippingof particles due to the dispersion treatment performed at the time ofpreparing a composition for forming a magnetic layer, for example. Theinventor of the present invention also assumes that the more the fineparticles contained in the magnetic layer, the further the hardness ofthe magnetic layer decreases, although the reason is unclear. In a casewhere the hardness of the magnetic layer decreases, the surface of themagnetic layer is scraped when the head slides on the surface of themagnetic layer (magnetic layer scraping), the foreign substancesoccurring due to the scraping are interposed between the surface of themagnetic layer and the head, and as a result, spacing loss occurs.

The inventor of the present invention considers that in theferromagnetic hexagonal ferrite powder present in the magnetic layer,the former particles are particles resulting in a diffraction peak inX-ray diffraction analysis using an In-Plane method, and the latterparticles do not result in a diffraction peak or exert a small influenceon a diffraction peak because they are fine. Therefore, the inventor ofthe present invention assumes that based on the intensity of thediffraction peak determined by X-ray diffraction analysis performed onthe magnetic layer by using the In-Plane method, the way the particles,which support the abrasive pushed into the inside of the magnetic layerand exert an influence on the degree of permeation of the abrasive, arepresent in the magnetic layer can be controlled, and as a result, thedegree of permeation of the abrasive can be controlled. The inventor ofthe present invention considers that the XRD intensity ratio, which willbe specifically described later, is a parameter relating to theaforementioned point.

Meanwhile, the squareness ratio in a vertical direction is a ratio ofremnant magnetization to saturation magnetization measured in adirection perpendicular to the surface of the magnetic layer. Thesmaller the remnant magnetization, the lower the ratio. Presumably, itis difficult for the latter particles to retain magnetization becausethey are fine. Therefore, presumably, as the amount of the latterparticles contained in the magnetic layer increases, the squarenessratio in a vertical direction tends to be reduced. Accordingly, theinventor of the present invention considers that the squareness ratio ina vertical direction can be a parameter of the amount of the fineparticles (the latter particles described above) present in the magneticlayer. The inventor of the present invention considers that as theamount of such fine particles contained in the magnetic layer increases,the hardness of the magnetic layer may decrease, and accordingly, thesurface of the magnetic layer may be scraped when the head slides on thesurface of the magnetic layer, the foreign substances that occur due tothe scraping may be interposed between the surface of the magnetic layerand the head, and hence the spacing loss strongly tends to occur.

The inventor of the present invention considers that in theaforementioned magnetic recording medium, each of the XRD intensityratio and the squareness ratio in a vertical direction is in theaforementioned range, and this makes a contribution to the inhibition ofthe deterioration of the electromagnetic conversion characteristicsduring the repeated sliding. According to the inventor of the presentinvention, presumably, this is because the control of the XRD intensityratio mainly makes it possible to inhibit the head scraping, and thecontrol of the squareness ratio in a vertical direction mainly makes itpossible to inhibit the magnetic layer scraping.

Furthermore, the inventor of the present invention considers that, inthe aforementioned magnetic recording medium, the state where thecoefficient of friction measured in a base material portion within asurface of the magnetic layer is equal to or lower than 0.30 makes acontribution of the further inhibition of the deterioration of theelectromagnetic conversion characteristics during the repeated sliding.This point will be further described below.

In the present invention and the present specification, “base materialportion” refers to a portion which is within the surface of the magneticlayer of the magnetic recording medium and specified by the followingmethod.

A surface, which has convex components and concave components found tohave the same volume within a visual field by being measured using anAtomic Force Microscope (AFM), is determined as a reference plane. Then,a bump having a height of equal to or greater than 15 nm from thereference plane is defined as a projection. Furthermore, a portion inwhich no such a projection is present, that is, a portion which iswithin the surface of the magnetic layer of the magnetic recordingmedium and in which a projection having a height of equal to or greaterthan 15 run from the reference plane is not detected is identified as abase material portion.

The coefficient of friction measured in the base material portion is avalue measured by the following method.

In the base material portion (measurement site: a site 10 μm long alonga longitudinal direction in a magnetic tape or a site extends 10 μmalong a radius direction in a magnetic disc), a spherical indenter madeof diamond having a radius of 1 μm is allowed to reciprocate once undera load of 100 μN and a speed of 1 μm/sec, and a frictional force(horizontal force) and a normal force are measured. Each of thefrictional force and the normal force measured herein is an arithmeticmean of values obtained by measuring at all times the frictional forceand the normal force while the indenter is reciprocating once. Themeasurement described above can be performed, for example, using aTI-950 TRIBOINDENTER manufactured by Hysitron. From the arithmetic meanof the measured frictional force and the arithmetic mean of the measurednormal force, a value μ of the coefficient of friction is calculated.The coefficient of friction is a value determined from a frictionalforce (horizontal force) F (unit: newton (N)) and a normal force N(unit: newton (N)) by an equation of F=μN. The aforementionedmeasurement and the calculation of the value μ of the coefficient offriction are performed for three sites within the base material portionthat are randomly determined within the surface of the magnetic layer ofthe magnetic recording medium, and the arithmetic mean of the obtainedthree measurement values is taken as the coefficient of frictionmeasured in the base material portion. Hereinafter, the coefficient offriction measured in the base material portion will be described as“base material friction” as well.

In recent years, incorporating non-magnetic powder such as an abrasiveinto a magnetic layer of a magnetic recording medium has become awide-spread process. Usually, by protruding from the surface of themagnetic layer and forming projections, the non-magnetic powder canperform various functions. Generally, a coefficient of friction measuredfor a magnetic recording medium is a coefficient of friction measuredwithin a region including such projections. In contrast, the basematerial friction is measured in a portion which is within the surfaceof the magnetic layer of the magnetic recording medium and in which aprojection having a height of equal to or greater than 15 nm from thereference plane is not detected. That is, the base material friction ismeasured in the base material portion. The base material portion isconsidered to infrequently come into contact with a head when the headslides on the surface of the magnetic layer. It is considered that in acase where the base material portion, which comes into contact with thehead even though the frequency is low, has a high coefficient offriction, the head is hindered to smoothly slide on the base materialportion. According to the inventor of the present invention, presumably,in a case where the head cannot smoothly slide on the base materialportion, the sliding between the surface of the magnetic layer and thehead may deteriorate, and as a result, the surface of the magnetic layermay be damaged, and the magnetic layer scraping may occur. On thecontrary, it is considered that the state where the base materialfriction in the magnetic recording medium is equal to or lower than 0.30makes a contribution to enable the head to smoothly slide on the basematerial portion, and as a result, the occurrence of magnetic layerscraping can be inhibited. The inventor of the present invention assumesthat the points described above may make it possible to inhibit theelectromagnetic conversion characteristics from deteriorating during therepeated sliding due to the occurrence of spacing loss resulting fromforeign substances generated by the magnetic layer scraping.

The points described so far are assumptions that the inventor of thepresent invention made regarding the mechanism which makes it possibleto inhibit the deterioration of the electromagnetic conversioncharacteristics in the magnetic recording medium even though the headrepeatedly slides on the surface of the magnetic layer. However, thepresent invention is not limited to the assumption. The presentspecification includes the assumption of the inventor of the presentinvention, and the present invention is not limited to the assumption.

Hereinbelow, various values will be more specifically described.

XRD Intensity Ratio

In the magnetic recording medium, the magnetic layer containsferromagnetic hexagonal ferrite powder. The XRD intensity ratio isdetermined by performing X-ray diffraction analysis on the magneticlayer containing the ferromagnetic hexagonal ferrite powder by using anIn-Plane method. Hereinafter, the X-ray diffraction analysis performedusing an In-Plane method will be described as “In-Plane XRD” as well.In-Plane XRD is performed by irradiating the surface of the magneticlayer with X-rays by using a thin film X-ray diffractometer under thefollowing conditions. Magnetic recording media are roughly classifiedinto a tape-like magnetic recording medium (magnetic tape) and adisc-like magnetic recording medium (magnetic disc). The magnetic tapeis measured in a longitudinal direction, and the magnetic disc ismeasured in a radius direction.

Radiation source used: Cu radiation (power of 45 kV, 200 mA)

Scan condition: 0.05 degree/step within a range of 20 to 40 degree, 0.1degree/min

Optical system used: parallel optical system

Measurement method: 2 θ_(χ) scan (X-ray incidence angle: 0.25°)

The above conditions are values set in the thin film X-raydiffractometer. As the thin film X-ray diffractometer, known instrumentscan be used. As one of the thin film X-ray diffiactometers, SmartLabmanufactured by Rigaku Corporation can be exemplified. The sample usedfor In-Plane XRD analysis is not limited in terms of the size and shape,as long as it is a medium sample which is cut from a magnetic recordingmedium to be measured and enables the confirmation of a diffraction peakwhich will be described later.

Examples of the techniques of X-ray diffraction analysis include thinfilm X-ray diffraction and powder X-ray diffraction. By the powder X-raydiffraction, the X-ray diffraction of a powder sample is measured. Incontrast, by the thin film X-ray diffraction, it is possible to measurethe X-ray diffraction of a layer formed on a substrate and the like. Thethin film X-ray diffraction is classified into an In-Plane method and anOut-Of-Plane method. In the Out-Of-Plane method, the X-ray incidenceangle during measurement is within a range of 5.00° to 90.00°. Incontrast, in the In-Plane method, the X-ray incidence angle is generallywithin a range of 0.20° to 0.50°. In the present invention and thepresent specification, the X-ray incidence angle in In-Plane XRD is setto be 0.25° as described above. In the In-Plane method, the X-rayincidence angle is smaller than in the Out-Of-Plane method, and hencethe X-ray permeation depth is small. Accordingly, by the X-raydiffraction analysis (In-Plane XRD) using the In-Plane method, it ispossible to analyze the X-ray diffraction of a surface layer portion ofa sample to be measured. For the sample of the magnetic recordingmedium, the X-ray diffraction of the magnetic layer can be analyzed byIn-Plane XRD. In an X-ray diffraction spectrum obtained by theaforementioned In-Plane XRD, the aforementioned XRD intensity ratio isan intensity ratio (Int (110)/Int (114)) of a peak intensity Int (110)of a diffraction peak of (110) plane of a crystal structure of thehexagonal ferrite to a peak intensity Int (114) of a diffraction peak of(114) plane of the crystal structure. Int is used as the abbreviation ofintensity. In the X-ray diffraction spectrum obtained by In-Plane XRD(ordinate: intensity, abscissa: diffraction angle 2 θ_(χ) (degree)), thediffraction peak of (114) plane is a peak detected at 2 θ_(χ) that iswithin a range of 33 to 36 degree, and the diffraction peak of (110)plane is a peak detected at 2 0x that is within a range of 29 to 32degree.

Among diffraction planes, (114) plane of the crystal structure of thehexagonal ferrite is positioned close to a direction of a magnetizationeasy axis (c-axis direction) of the particles of the ferromagnetichexagonal ferrite powder (hexagonal ferrite particles). The (110) planeof the hexagonal ferrite crystal structure is positioned in a directionorthogonal the direction of the magnetization easy axis.

Regarding the aforementioned former particles among the hexagonalferrite particles contained in the magnetic layer, the inventor of thepresent invention considered that the more the direction of theparticles orthogonal to the magnetization easy axis is parallel to thesurface of the magnetic layer, the more difficult it is for the abrasiveto permeate the inside of the magnetic layer by being supported by thehexagonal ferrite particles. In contrast, regarding the former particlesin the magnetic layer, the inventor of the present invention considersthat the more the direction of the particles orthogonal to themagnetization easy axis is perpendicular to the surface of the magneticlayer, the easier it is for the abrasive to permeate the inside of themagnetic layer because it is difficult for the abrasive to be supportedby the hexagonal ferrite powder. Furthermore, the inventor of thepresent invention assumes that in the X-ray diffraction spectradetermined by In-Plane XRD, in a case where the intensity ratio (Int(110)/Int (114); XRD intensity ratio) of the peak intensity Int (110) ofthe diffraction peak of (110) plane to the peak intensity Int (114) ofthe diffraction peak of (114) plane of the hexagonal ferrite crystalstructure is high, it means that the magnetic layer contains a largeamount of the former particles whose direction orthogonal to thedirection of the magnetization easy axis is more parallel to the surfaceof the magnetic layer; and in a case where the XRD intensity ratio islow, it means that the magnetic layer contains a small amount of suchformer particles. In addition, the inventor considers that in a casewhere the XRD intensity ratio is equal to or lower than 4.0, it meansthat the former particles, that is, the particles, which support theabrasive pushed into the inside of the magnetic layer and exert aninfluence on the degree of the permeation of the abrasive, merelysupport the abrasive, and as a result, the abrasive can appropriatelypermeate the inside of the magnetic layer at the time when a head slideson the surface of the magnetic layer. The inventor of the presentinvention assumes that the aforementioned mechanism may make acontribution to hinder the occurrence of the head scraping even thoughthe head repeatedly slides on the surface of the magnetic layer. Incontrast, the inventor of the present invention considers that the statein which the abrasive appropriately protrudes from the surface of themagnetic layer when the head slides on the surface of the magnetic layermay make a contribution to the reduction of the contact area (realcontact) between the surface of the magnetic layer and the head. Theinventor considers that the larger the real contact area, the strongerthe force applied to the surface of the magnetic layer from the headwhen the head slides on the surface of the magnetic layer, and as aresult, the surface of the magnetic layer is damaged and scraped.Regarding this point, the inventor of the present invention assumes thatin a case where the XRD intensity ratio is equal to or higher than 0.5,it shows that the aforementioned former particles are present in themagnetic layer in a state of being able to support the abrasive withallowing the abrasive to appropriately protrude from the surface of themagnetic layer when the head slides on the surface of the magneticlayer.

From the viewpoint of further inhibiting the deterioration of theelectromagnetic conversion characteristics, the XRD intensity ratio ispreferably equal to or lower than 3.5, and more preferably equal to orlower than 3.0. From the same viewpoint, the XRD intensity ratio ispreferably equal to or higher than 0.7, and more preferably equal to orhigher than 1.0. The XRD intensity ratio can be controlled by thetreatment conditions of the alignment treatment performed in themanufacturing process of the magnetic recording medium. As the alignmenttreatment, it is preferable to perform a vertical alignment treatment.The vertical alignment treatment can be preferably performed by applyinga magnetic field in a direction perpendicular to a surface of the wet(undried) coating layer of the composition for forming a magnetic layer.The further the alignment conditions are strengthened, the higher theXRD intensity ratio tends to be. Examples of the treatment conditions ofthe alignment treatment include the magnetic field intensity in thealignment treatment and the like. The treatment conditions of thealignment treatment are not particularly limited, and may be set suchthat an XRD intensity ratio of equal to or higher than 0.5 and equal toor lower than 4.0 can be achieved. For example, the magnetic fieldintensity in the vertical alignment treatment can be set to be 0.10 to0.80 T or 0.10 to 0.60 T. As the dispersibility of the ferromagnetichexagonal ferrite powder in the composition for forming a magnetic layeris improved, the value of the XRD intensity ratio tends to increase bythe vertical alignment treatment.

Squareness Ratio in Vertical Direction

The squareness ratio in a vertical direction is a squareness ratiomeasured in a vertical direction of the magnetic recording medium.“Vertical direction” described regarding the squareness ratio refers toa direction orthogonal to the surface of the magnetic layer. Forexample, in a case where the magnetic recording medium is a tape-likemagnetic recording medium, that is, a magnetic tape, the verticaldirection is a direction orthogonal to a longitudinal direction of themagnetic tape. The squareness ratio in a vertical direction is measuredusing a vibrating sample fluxmeter. Specifically, in the presentinvention and the present specification, the squareness ratio in avertical direction is a value determined by carrying out scanning in thevibrating sample fluxmeter by applying a maximum external magnetic fieldof 1,194 kA/m (15 kOe) as an external magnetic field to the magneticrecording medium, at a measurement temperature of 23° C.±1° C. under thecondition of a scan rate of 4.8 kA/m/sec (60 Oe/sec), which is usedafter being corrected for a demagnetizing field. The measured squarenessratio is a value from which the magnetization of a sample probe of thevibrating sample fluxmeter is subtracted as background noise.

The squareness ratio in a vertical direction of the magnetic recordingmedium is equal to or higher than 0.65. The inventor of the presentinvention assumes that the squareness ratio in a vertical direction ofthe magnetic recording medium can be a parameter of the amount of theaforementioned latter particles (fine particles) present in the magneticlayer that are considered to induce the reduction in the hardness of themagnetic layer. It is considered that the magnetic layer in the magneticrecording medium having a squareness ratio in a vertical direction ofequal to or higher than 0.65 has high hardness because of containing asmall amount of such fine particles and is hardly scraped by the slidingof the head on the surface of the magnetic layer. Presumably, becausethe surface of the magnetic layer is hardly scraped, it is possible toinhibit the electromagnetic conversion characteristics fromdeteriorating due to the occurrence of spacing loss resulting fromforeign substances that occur due to the scraping of the surface of themagnetic layer. From the viewpoint of further inhibiting thedeterioration of the electromagnetic conversion characteristics, thesquareness ratio in a vertical direction is preferably equal to orhigher than 0.68, more preferably equal to or higher than 0.70, evenmore preferably equal to or higher than 0.73, and still more preferablyequal to or higher than 0.75. In principle, the squareness ratio is 1.00at most. Accordingly, the squareness ratio in a vertical direction ofthe magnetic recording medium is equal to or lower than 1.00. Thesquareness ratio in a vertical direction may be equal to or lower than0.95, 0.90, 0.87, or 0.85, for example. The larger the value of thesquareness ratio in a vertical direction, the smaller the amount of theaforementioned fine latter particles in the magnetic layer. Therefore,it is considered that from the viewpoint of the hardness of the magneticlayer, the value of the squareness ratio is preferably large.Accordingly, the squareness ratio in a vertical direction may be higherthan the upper limit exemplified above.

The inventor of the present invention considers that in order to obtaina squareness ratio in a vertical direction of equal to or higher than0.65, it is preferable to inhibit fine particles from occurring due topartial chipping of particles in the step of preparing the compositionfor forming a magnetic layer. Specific means for inhibiting theoccurrence of chipping will be described later.

Base Material Friction

The coefficient of friction (base material friction) measured in thebase material portion within the surface of the magnetic layer of themagnetic recording medium is equal to or lower than 0.30, from theviewpoint of inhibiting the deterioration of the electromagneticconversion characteristics. From the viewpoint of further inhibiting thedeterioration of the electromagnetic conversion characteristics, thecoefficient of friction is preferably equal to or lower than 0.28, andmore preferably equal to or lower than 0.26. Furthermore, the basematerial friction can be, for example, equal to or higher than 0.10,0.15, or 0.20. Here, from the viewpoint of inhibiting the deteriorationof the electromagnetic conversion characteristics, the lower the basematerial friction, the more preferable. Therefore, the base materialfriction may be lower than the value exemplified above.

In the method for measuring the base material friction, a bump having aheight of equal to or greater than 15 nm from the reference plane isdefined as a projection. This is because, generally, the bump recognizedas a projection present on the surface of the magnetic layer is mainly abump having a height of equal to or greater than 15 nm from thereference plane. Such a projection is formed of non-magnetic powder suchas an abrasive on the surface of the magnetic layer. In contrast, it isconsidered that on the surface of the magnetic layer, there may bemicroscopic concavities and convexities smaller than concavities andconvexities formed by such projections. Furthermore, presumably, bycontrolling the shape of such microscopic concavities and convexities,it is possible to adjust the base material friction. For example, as oneof the means for adjusting the base material friction, two or more kindsof ferromagnetic hexagonal ferrite powders having different meanparticle sizes are used as ferromagnetic hexagonal ferrite powder. Morespecifically, it is considered that in a case where the ferromagnetichexagonal ferrite powder having a larger mean particle size becomesconvex portions, the aforementioned microscopic concavities andconvexities can be formed in the base material portion, and in a casewhere a mixing ratio of the ferromagnetic hexagonal ferrite powderhaving a larger mean particle size is increased, the ratio of the convexportions present in the base material portions can be increased (orinversely, in a case where the mixing ratio is decreased, the ratio ofthe convex portions present in the base material portion can bedecreased). Means for doing these things will be specifically describedlater.

As another means, for example, in addition to the non-magnetic powdersuch as an abrasive, which can form a projection having a height ofequal to or greater than 15 nm from the reference plane on the surfaceof the magnetic layer, another non-magnetic powder having a meanparticle size larger than that of the ferromagnetic hexagonal ferritepowder is used to form the magnetic layer. More specifically, it isconsidered that in a case where the aforementioned another non-magneticpowder becomes convex portions, the aforementioned microscopicconcavities and convexities can be formed in the base material portion,and in a case where the mixing ratio of the aforementioned anothernon-magnetic powder is increased, the ratio of the convex portionspresent in the base material portion can be increased (or inversely, ina case where the mixing ratio is decreased, the ratio of the convexportions present in the base material portion can be decreased). Meansfor doing these things will be specifically described later.

In addition, by combining two kinds of means described above, the basematerial friction can be adjusted.

Here, the aforementioned adjustment means is merely an example. The basematerial friction can become equal to or lower than 0.30 by any meansthat can adjust the base material friction, and this aspect is alsoincluded in the present invention.

Hereinafter, the magnetic recording medium will be more specificallydescribed.

Magnetic Layer

Ferromagnetic Hexagonal Ferrite Powder

The magnetic layer of the magnetic recording medium containsferromagnetic hexagonal ferrite powder as ferromagnetic powder.Regarding the ferromagnetic hexagonal ferrite powder, a magnetoplumbitetype (referred to as “M type” as well), a W type, a Y type, and Z typeare known as crystal structures of the hexagonal ferrite. Theferromagnetic hexagonal ferrite powder contained in the magnetic layermay take any of the above crystal structures. The crystal structures ofthe hexagonal ferrite contain an iron atom and a divalent metal atom asconstituent atoms. The divalent metal atom is a metal atom which canbecome a divalent cation as an ion, and examples thereof include alkaliearth metal atoms such as a barium atom, a strontium atom, and a calciumatom, a lead atom, and the like. For example, the hexagonal ferritecontaining a barium atom as a divalent metal atom is barium ferrite, andthe hexagonal ferrite containing a strontium atom is strontium ferrite.The hexagonal ferrite may be a mixed crystal of two or more kinds ofhexagonal ferrite. As one of the mixed crystals, a mixed crystal ofbarium ferrite and strontium ferrite can be exemplified.

As described above, as one of the means for adjusting the base materialfriction, for example, as ferromagnetic hexagonal ferrite powder, two ormore kinds of ferromagnetic hexagonal ferrite powders having differentmean particle sizes are used to form the magnetic layer. In this case,among the two or more kinds of ferromagnetic hexagonal ferrite powders,the ferromagnetic hexagonal ferrite powder having a small mean particlesize is preferably used as ferromagnetic hexagonal ferrite powder thatis used at the highest proportion, because then the recording density ofthe magnetic recording medium is improved. In this respect, in a casewhere two or more kinds of ferromagnetic hexagonal ferrite powdershaving different mean particle sizes are incorporated into the magneticlayer, as the ferromagnetic hexagonal ferrite powder accounting for thehighest proportion based on mass, ferromagnetic hexagonal ferrite powderhaving a mean particle size of equal to or smaller than 50 nm ispreferably used. In contrast, from the viewpoint of the magnetizationstability, the mean particle size of the ferromagnetic hexagonal ferritepowder accounting for the highest proportion is preferably equal to orgreater than 10 nm. In a case where one kind of ferromagnetic hexagonalferrite powder is used instead of two or more kinds of ferromagnetichexagonal ferrite powders having different mean particle sizes, for theaforementioned reason, the mean particle size of the ferromagnetichexagonal ferrite powder is preferably equal to or greater than 10 nmand equal to or smaller than 50 nm.

In contrast, it is preferable that the ferromagnetic hexagonal ferritepowder used together with the ferromagnetic hexagonal ferrite powderaccounting for the highest proportion has a mean particle size largerthan that of the ferromagnetic hexagonal ferrite powder accounting forthe highest proportion. This is because the base material friction isconsidered to be able to be reduced by the convex portions formed in thebase material portion by the ferromagnetic hexagonal ferrite powderhaving a large mean particle size. In this respect, for the meanparticle size of the ferromagnetic hexagonal ferrite powder accountingfor the highest proportion and the mean particle size of theferromagnetic hexagonal ferrite powder used together with theaforementioned ferromagnetic hexagonal ferrite powder, a differenceobtained by “(mean particle size of the latter)−(mean particle size ofthe former)” is preferably within a range of 10 to 80 nm, morepreferably within a range of 10 to 50 nm, and even more preferablywithin a range of 10 to 40 nm. It goes without saying that two or moreferromagnetic hexagonal ferrite powder having different mean particlesizes can be used as the ferromagnetic hexagonal ferrite powder usedtogether with the ferromagnetic hexagonal ferrite powder accounting forthe highest proportion. In this case, the mean particle size of at leastone kind of ferromagnetic hexagonal ferrite powder among theaforementioned two or more kinds of ferromagnetic hexagonal ferritepowders preferably satisfies the aforementioned difference with respectto the mean particle size of the ferromagnetic hexagonal ferrite powderused at the highest proportion, the mean particle size of more kinds offerromagnetic hexagonal ferrite powders more preferably satisfies theaforementioned difference with respect to the mean particle size of theferromagnetic hexagonal ferrite powder used at the highest proportion,and the mean particle size of all ferromagnetic hexagonal ferritepowders even more preferably satisfies the mean particle size of theferromagnetic hexagonal ferrite powder used at the highest proportion.

Regarding the two or more kinds of ferromagnetic hexagonal ferritepowders having different mean particle sizes, from the viewpoint ofcontrolling the base material friction, a mixing ratio between theferromagnetic hexagonal ferrite powder accounting for the highestproportion and another ferromagnetic hexagonal ferrite powder (in a casewhere two or more kinds having different mean particle sizes are used asanother ferromagnetic hexagonal ferrite powder, the total amountthereof) based on mass is preferably within a range of 90.0:10.0 to99.9:0.1 (the former:the latter), and more preferably within a range of95.0:5.0 to 99.5:0.5.

The ferromagnetic hexagonal ferrite powders having different meanparticle sizes refers to all or a portion of lots of ferromagnetichexagonal ferrite powders having different mean particle sizes. In acase where a number-based particle size distribution or a volume-basedparticle size distribution of the ferromagnetic hexagonal ferrite powdercontained in the magnetic layer of the magnetic recording medium formedusing the ferromagnetic hexagonal ferrite powders having different meanparticle sizes is measured by a known measurement method such as adynamic light scattering method or a laser diffraction method, in aparticle size distribution curve obtained by the measurement, generally,a peak can be confirmed in the mean particle size of the ferromagnetichexagonal ferrite powder used at the highest proportion or in thevicinity of the mean particle size. Furthermore, a peak is confirmed inthe mean particle size of each ferromagnetic hexagonal ferrite powder orin the vicinity thereof in some cases. Accordingly, for example, in acase where the particle size distribution of ferromagnetic hexagonalferrite powder, which is contained in the magnetic layer of the magneticrecording medium formed using ferromagnetic hexagonal ferrite powderhaving a mean particle size of 10 to 50 nm at the highest proportion, ismeasured, usually, in the particle size distribution curve, a maximumpeak can be confirmed within the range of a particle size of 10 to 50nm.

A portion of the aforementioned another ferromagnetic hexagonal ferritepowder may be substituted with still another non-magnetic powder whichwill be described later.

In the present invention and the present specification, unless otherwisespecified, the mean particle size of various powders such asferromagnetic hexagonal ferrite powder is a value measured using atransmission electron microscope by the following method.

The powder is imaged using a transmission electron microscope at a100,000× magnification, and the image is printed on photographic papersuch that the total magnification thereof becomes 500,000×, therebyobtaining an image of particles constituting the powder. From theobtained image of the particles, target particles are selected, theoutlines of the particles are traced using a digitizer, and the size ofthe particles (primary particles) is measured. The primary particlesrefer to independent particles not being aggregated with each other.

The measurement is performed for 500 particles that are randomlyextracted. The arithmetic mean of the particles sizes of the 500particles obtained as above is taken as the mean particle size of thepowder. As the aforementioned transmission electron microscope, forexample, it is possible to use a transmission electron microscope H-9000manufactured by Hitachi High-Technologies Corporation. Furthermore, theparticle size can be measured using known image analysis software suchas image analysis software KS-400 manufactured by Carl Zeiss AG.

In the present invention and the present specification, unless otherwisespecified, the mean particle size of the ferromagnetic hexagonal ferritepowder and another powder refers to a mean particle size determined bythe method described above. Unless otherwise specified, the meanparticle size shown in examples, which will be described later, is avalue measured using a transmission electron microscope H-9000manufactured by Hitachi High-Technologies Corporation as a transmissionelectron microscope and image analysis software KS-400 manufactured byCarl Zeiss AG as image analysis software.

As the method for collecting sample powder from a magnetic tape formeasuring the particle size, for example, it is possible to adopt themethod described in paragraph “0015” in JP2011-048878A.

In the present invention and present specification, unless otherwisespecified, the size of particles (particle size) constituting powder isrepresented by the following ones.

(1) In a case where the particles observed in the aforementioned imageof particles have a needle shape, a spindle shape, a columnar shape(here, the height is larger than the maximum major axis of the bottomsurface), or the like, the particle size is represented by the length ofthe major axis constituting the particles, that is, the major axislength.

(2) In a case where the particles observed in the aforementioned imageof particles have a plate-like shape or a columnar shape (here, thethickness or the height is smaller than the maximum major axis of theplate surface or the bottom surface), the particle size is representedby the maximum major axis of the plate surface or the bottom surface.

(3) In a case where the particles observed in the aforementioned imageof particles have a spherical shape, a polyhedral shape, an unspecifiedshape, or the like, and the major axis constituting the particles cannotbe specified from the shape, the particle size is represented by theequivalent circle diameter. The equivalent circle diameter refers to avalue determined by a circular projection method.

For obtaining the average aspect ratio of the powder, the length of theminor axis of the particles, that is, the minor axis length is measuredby the aforementioned measurement, and the value of (major axislength/minor axis length) is determined for each particle. The averageaspect ratio refers to the arithmetic mean of the values obtained forthe aforementioned 500 particles. Herein, unless otherwise specified, ina case where the particle size is defined by (1), the minor axis lengthrefers to the length of the minor axis constituting the particles; in acase where the particle size is defined by (2), the minor axis lengthrefers to the thickness or the height respectively; and in a case wherethe particle size is defined by (3), for convenience, the value of(major axis length/minor axis length) is regarded as 1 because thedistinction between a major axis and a minor axis is not applied to thiscase.

Furthermore, unless otherwise specified, in a case where the particleshave a specific shape, for example, in a case where the particle size isdefined by (1), the mean particle size is the average major axis length,and in a case where the particle size is defined by (2), the meanparticle size is the average plate diameter. In a case where theparticle size is defined by (3), the mean particle size is the averagediameter (also called the mean particle size or the mean particlediameter).

As described above, the shape of the particles constituting theferromagnetic hexagonal ferrite powder is specified by tracing theoutlines of the particles (primary particles) by using a digitizer inthe particle image obtained using a transmission electron microscope.Regarding the shape of the particles constituting the ferromagnetichexagonal ferrite powder, “plate-like” means a shape having two platesurfaces facing each other. Among particle shapes that do not have suchplate surfaces, a shape having a major axis and a minor axis differentfrom each other is “elliptical”. The major axis is an axis (straightline) which is the longest diameter of a particle. The minor axis is astraight line which is the longest diameter of a particle in a directionorthogonal to the major axis. A shape in which the major axis and theminor axis are the same as each other, that is, a shape in which themajor axis length equals the minor axis length is “spherical”. A shapein which the major axis and the minor axis cannot be identified iscalled “amorphous”. The imaging performed for identifying the particleshape by using a transmission electron microscope is carried out withoutperforming an alignment treatment on the powder to be imaged. The rawmaterial powder used for preparing the composition for forming amagnetic layer and the ferromagnetic hexagonal ferrite powder containedin the magnetic layer may take any of the plate-like shape, theelliptical shape, the spherical shape and the amorphous shape.

For details of the ferromagnetic hexagonal ferrite powder, for example,paragraphs “0134” to “0136” in JP2011-216149A can also be referred to.

The content (filling rate) of the ferromagnetic hexagonal ferrite powderin the magnetic layer is preferably within a range of 50% to 90% bymass, and more preferably within a range of 60% to 90% by mass. Themagnetic layer contains at least a binder and an abrasive as componentsother than the ferromagnetic hexagonal ferrite powder, and canoptionally contain one or more kinds of additives. From the viewpoint ofimproving the recording density, the filling rate of the ferromagnetichexagonal ferrite powder in the magnetic layer is preferably high.

Binder and Curing Agent

The magnetic layer of the magnetic recording medium contains a binder.As the binder, one or more kinds of resins are used. The resin may be ahomopolymer or a copolymer. As the binder contained in the magneticlayer, a binder selected from an acryl resin obtained by copolymerizinga polyurethane resin, a polyester resin, a polyamide resin, a vinylchloride resin, styrene, acrylonitrile, or methyl methacrylate, acellulose resin such as nitrocellulose, an epoxy resin, a phenoxy resin,a polyvinyl alkyral resin such as polyvinyl acetal or polyvinyl butyralcan be used singly, or a plurality of resins can be used by being mixedtogether. Among these, a polyurethane resin, an acryl resin, a celluloseresin, and a vinyl chloride resin are preferable. These resins can beused as a binder in a non-magnetic layer and/or a back coating layerwhich will be described later. Regarding the aforementioned binders,paragraphs “0029” to “0031” in JP2010-24113A can be referred to. Theaverage molecular weight of the resin used as a binder can be equal toor greater than 10,000 and equal to or less than 200,000 in terms of aweight-average molecular weight, for example. The weight-averagemolecular weight in the present invention and the present specificationis a value determined by measuring a molecular weight by gel permeationchromatography (GPC) and expressing the molecular weight in terms ofpolystyrene. As the measurement conditions, the following conditions canbe exemplified. The weight-average molecular weight shown in exampleswhich will be described later is a value determined by measuring amolecular weight under the following measurement conditions andexpressing the molecular weight in terms of polystyrene.

GPC instrument: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (Inner Diameter)×30.0 cm)

Eluent: tetrahydrofuran (THF)

At the time of forming the magnetic layer, it is possible to use acuring agent together with a resin usable as the aforementioned binder.In an aspect, the curing agent can be a thermosetting compound which isa compound experiencing a curing reaction (crosslinking reaction) byheating. In another aspect, the curing agent can be a photocurablecompound experiencing a curing reaction (crosslinking reaction) by lightirradiation. The curing agent experiences a curing reaction in themanufacturing process of the magnetic recording medium. In this way, atleast a portion of the curing agent can be contained in the magneticlayer, in a state of reacting (cross-linked) with other components suchas the binder. The curing agent is preferably a thermosetting compoundwhich is suitably polyisocyanate. For details of polyisocyanate,paragraphs “0124” and “0125” in JP2011-216149A can be referred to. Thecuring agent can be used by being added to the composition for forming amagnetic layer, in an amount of 0 to 80.0 parts by mass with respect to100.0 parts by mass of the binder and preferably in an amount of 50.0 to80.0 parts by mass from the viewpoint of improving the hardness of themagnetic layer.

Abrasive

The magnetic layer of the magnetic recording medium contains anabrasive. The abrasive refers to non-magnetic powder having a Mohshardness of higher than 8, and is preferably non-magnetic powder havinga Mohs hardness of equal to or higher than 9. The abrasive may be powderof an inorganic substance (inorganic powder) or powder of an organicsubstance (organic powder), and is preferably inorganic powder. Theabrasive is preferably inorganic powder having a Mohs hardness of higherthan 8, and even more preferably inorganic powder having Mohs hardnessof equal to or higher than 9. The maximum value of the Mohs hardness is10 which is the Mohs hardness of diamond. Specific examples of theabrasive include powder of alumina (Al₂O₃), silicon carbide, boroncarbide (B₄C), TiC, cerium oxide, zirconium oxide (ZrO₂), diamond, andthe like. Among these, alumina powder is preferable. Regarding thealumina powder, paragraph “0021” in JP2013-229090A can also be referredto. As a parameter of the particle size of the abrasive, specificsurface area can be used. The larger the specific surface area, thesmaller the particle size. It is preferable to use an abrasive having aspecific surface area (hereinafter, described as “BET specific surfacearea”) of equal to or greater than 14 m²/g, which is measured forprimary particles by a Brunauer-Emmett-Teller (BET) method. From theviewpoint of dispersibility, it is preferable to use an abrasive havinga BET specific surface area of equal to or less than 40 m²/g. Thecontent of the abrasive in the magnetic layer is preferably 1.0 to 20.0parts by mass with respect to 100.0 parts by mass of the ferromagnetichexagonal ferrite powder.

Additive

The magnetic layer contains the ferromagnetic hexagonal ferrite powder,the binder, and the abrasive, and may further contain one or more kindsof additives if necessary. As one of the additives, the aforementionedcuring agent can be exemplified. Examples of the additives that can becontained in the magnetic layer include non-magnetic powder, alubricant, a dispersant, a dispersion aid, a fungicide, an antistaticagent, an antioxidant, and the like. As one of the additives which canbe used in the magnetic layer containing the abrasive, the dispersantdescribed in paragraphs “0012” to “0022” in JP2013-131285A can beexemplified as a dispersant for improving the dispersibility of theabrasive in the composition for forming a magnetic layer.

Examples of the dispersant also include known dispersants such as acarboxy group-containing compound and a nitrogen-containing compound.The nitrogen-containing compound may be any one of a primary aminerepresented by NH₂R, a secondary amine represented by NHR₂, and atertiary amine represented by NR₃, for example. R represents anystructure constituting the nitrogen-containing compound, and a pluralityof R′s present in the compound may be the same as or different from eachother. The nitrogen-containing compound may be a compound (polymer)having a plurality of repeating structures in a molecule. The inventorof the present invention considers that because the nitrogen-containingportion of the nitrogen-containing compound functions as a portionadsorbed onto the surface of particles of the ferromagnetic hexagonalferrite powder, the nitrogen-containing compound can act as adispersant. Examples of the carboxy group-containing compound includefatty acids such as oleic acid. Regarding the carboxy group-containingcompound, the inventor of the present invention considers that becausethe carboxy group functions as a portion adsorbed onto the surface ofparticles of the ferromagnetic hexagonal ferrite powder, the carboxygroup-containing compound can act as a dispersant. It is also preferableto use the carboxy group-containing compound and the nitrogen-containingcompound in combination.

Examples of the non-magnetic powder that can be contained in themagnetic layer include non-magnetic powder (hereinafter, described as“projection-forming agent” as well) which can contribute to the controlof frictional characteristics by forming projections on the surface ofthe magnetic layer. As such a non-magnetic powder, it is possible to usevarious non-magnetic powders generally used in a magnetic layer. Thenon-magnetic powder may be inorganic powder or organic powder. In anaspect, from the viewpoint of uniformizing the frictionalcharacteristics, it is preferable that the particle size distribution ofthe non-magnetic powder is not polydisperse distribution having aplurality of peaks in the distribution but monodisperse distributionshowing a single peak. From the viewpoint of ease of availability of themonodisperse particles, the non-magnetic powder is preferably inorganicpowder. Examples of the inorganic powder include powder of a metaloxide, a metal carbonate, a metal sulfate, a metal nitride, a metalcarbide, a metal sulfide, and the like. The particles constituting thenon-magnetic powder are preferably colloidal particles, and morepreferably colloidal particles of an inorganic oxide. From the viewpointof ease of availability of the monodisperse particles, the inorganicoxide constituting the colloidal particles of an inorganic oxide ispreferably silicon dioxide (silica). The colloidal particles of aninorganic oxide are preferably colloidal silica (colloidal silicaparticles). In the present invention and the present specification,“colloidal particles” refer to the particles which can form a colloidaldispersion by being dispersed without being precipitated in a case wherethe particles are added in an amount of 1 g per 100 mL of at least oneorganic solvent among methyl ethyl ketone, cyclohexanone, toluene, ethylacetate, and a mixed solvent containing two or more kinds of thesolvents described above at any mixing ratio. In another aspect, thenon-magnetic powder is also preferably carbon black. The mean particlesize of the non-magnetic powder is 30 to 300 nm for example, andpreferably 40 to 200 nm. The content of the non-magnetic powder in themagnetic layer is, with respect to 100.0 parts by mass of theferromagnetic hexagonal ferrite powder, preferably 1.0 to 4.0 parts bymass and more preferably 1.5 to 3.5 parts by mass, because then thenon-magnetic filler can demonstrate better the function thereof.

As described above, in order to control the base material friction suchthat it becomes equal to or lower than 0.30, in addition to theaforementioned non-magnetic powder, another non-magnetic powder can alsobe used. The Mohs hardness of such a non-magnetic powder is preferablyequal to or lower than 8, and various non-magnetic powders generallyused in a non-magnetic layer can be used. The details thereof will bedescribed later in relation to the non-magnetic layer. As more preferrednon-magnetic powder, colcothar can be exemplified. The Mohs hardness ofcolcothar is about 6.

It is preferable that the aforementioned another non-magnetic powder hasa mean particle size larger than that of ferromagnetic hexagonal ferritepowder, just like the ferromagnetic hexagonal ferrite powder usedtogether with the ferromagnetic hexagonal ferrite powder described aboveof which the proportion in the magnetic layer is the highest. This isbecause the convex portions formed in the base material portion by theaforementioned another non-magnetic powder are considered to be able toreduce the base material friction. In this respect, regarding the meanparticle size of the ferromagnetic hexagonal ferrite powder and the meanparticle size of the aforementioned another non-magnetic powder usedtogether with the ferromagnetic hexagonal ferrite powder, a differencedetermined by “(mean particle size of the latter)−(mean particle size ofthe former)” is preferably within a range of 10 to 80 nm, and morepreferably within a range of 10 to 50 nm. In a case where two or morekinds of ferromagnetic hexagonal ferrite powders having different meanparticle sizes are used as ferromagnetic hexagonal ferrite powder, asthe ferromagnetic hexagonal ferrite powder used for calculating thedifference in the mean particle size with respect to the aforementionedanother non-magnetic hexagonal ferrite powder, among two or more kindsof ferromagnetic hexagonal ferrite powders, ferromagnetic hexagonalferrite powder accounting for the highest proportion based on mass isadopted. It goes without saying that two or more kinds of non-magneticpowders having different mean particle sizes can be used as theaforementioned another non-magnetic powder. In this case, the meanparticle size of at least one kind of non-magnetic powder among theaforementioned two or more kinds of non-magnetic powders preferablysatisfies the aforementioned difference with respect to the meanparticle size of the ferromagnetic hexagonal ferrite powder, the meanparticle size of more kinds of non-magnetic powders more preferablysatisfies the aforementioned difference with respect to the meanparticle size of the ferromagnetic hexagonal ferrite powder, and themean particle size of all non-magnetic powders even more preferablysatisfies the aforementioned difference with respect to the meanparticle size of the ferromagnetic hexagonal ferrite powder.

From the viewpoint of controlling the base material friction, a mixingratio between the ferromagnetic hexagonal ferrite powder and theaforementioned another non-magnetic powder (in a case where two or morekinds having different mean particle sizes are used as theaforementioned another non-magnetic powder, the total amount thereof)based on mass is preferably within a range of 90.0:10.0 to 99.9:0.1 (theformer:the latter), and more preferably within a range of 95.0:5.0 to99.5:0.5.

As various additives that can be optionally contained in the magneticlayer, commercially available products or those manufactured by knownmethods can be selected and used according to the desired properties.

The magnetic layer described so far can be provided on the surface ofthe non-magnetic support, directly or indirectly through a non-magneticlayer.

Non-Magnetic Layer

Next, a non-magnetic layer will be described.

The magnetic recording medium may have the magnetic layer directly onthe surface of the non-magnetic support, or may have a non-magneticlayer containing non-magnetic powder and a binder between thenon-magnetic support and the magnetic layer. The non-magnetic powdercontained in the non-magnetic layer may be inorganic powder or organicpowder. Furthermore, carbon black or the like can also be used. Examplesof the inorganic powder include powder of a metal, a metal oxide, ametal carbonate, a metal sulfate, a metal nitride, a metal carbide, ametal sulfide, and the like. These non-magnetic powders can be obtainedas commercially available products, or can be manufactured by knownmethods. For details of the non-magnetic powder, paragraphs “0036” to“0039” in JP2010-24113A can be referred to. The content (filling rate)of the non-magnetic powder in the non-magnetic layer is preferablywithin a range of 50% to 90% by mass, and more preferably within a rangeof 60% to 90% by mass.

For other details of the binder, the additives, and the like of thenon-magnetic layer, known techniques relating to the non-magnetic layercan be applied. For example, regarding the type and content of thebinder, the type and content of the additives, and the like, knowntechniques relating to the magnetic layer can also be applied.

In the present invention and the present specification, the non-magneticlayer also includes a substantially non-magnetic layer which containsnon-magnetic powder with a small amount of ferromagnetic powder as animpurity or by intention, for example. Herein, the substantiallynon-magnetic layer refers to a layer having a remnant flux density ofequal to or lower than 10 mT or a coercive force of equal to or lowerthan 7.96 kA/m (100 Oe) or having a remnant flux density of equal to orlower than 10 mT and a coercive force of equal to or lower than 7.96kA/m (100 Oe). It is preferable that the non-magnetic layer does nothave remnant flux density and coercive force.

Non-Magnetic Support

Next, a non-magnetic support (hereinafter, simply described as “support”as well) will be described.

Examples of the non-magnetic support include known supports such asbiaxially oriented polyethylene terephthalate, polyethylene naphthalate,polyamide, polyamide imide, and aromatic polyamide. Among these,polyethylene terephthalate, polyethylene naphthalate, and polyamide arepreferable. These supports may be subjected to corona discharge, aplasma treatment, an easy adhesion treatment, a heat treatment, and thelike in advance.

Back Coating Layer

The magnetic recording medium can have a back coating layer containingnon-magnetic powder and a binder, on a surface side of the non-magneticsupport opposite to a surface side provided with the magnetic layer. Itis preferable that the back coating layer contains either or both ofcarbon black and inorganic powder. Regarding the binder contained in theback coating layer and various additives which can be optionallycontained therein, known techniques relating to the back coating layercan be applied, and known techniques relating to the formulation of themagnetic layer and/or the non-magnetic layer can also be applied.

Various Thicknesses

The thickness of the non-magnetic support and each layer in the magneticrecording medium will be described below.

The thickness of the non-magnetic support is 3.0 to 80.0 μm for example,preferably 3.0 to 50.0 μm, and more preferably 3.0 to 10.0 μm.

The thickness of the magnetic layer can be optimized according to thesaturation magnetization of the magnetic head to be used, the length ofhead gap, the band of recording signals, and the like. The thickness ofthe magnetic layer is generally 10 nm to 100 rim. From the viewpoint ofhigh-density recording, the thickness of the magnetic layer ispreferably 20 to 90 nm, and more preferably 30 to 70 nm. The magneticlayer may be constituted with at least one layer, and may be separatedinto two or more layers having different magnetic characteristics.Furthermore, the constitution relating to known multi-layered magneticlayers can be applied. In a case where the magnetic layer is separatedinto two or more layers, the thickness of the magnetic layer means thetotal thickness of the layers.

The thickness of the non-magnetic layer is equal to or greater than 50nm for example, preferably equal to or greater than 70 nm, and morepreferably equal to or greater than 100 nm. In contrast, the thicknessof the non-magnetic layer is preferably equal to or less than 800 nm,and more preferably equal to or less than 500 nm.

The thickness of the back coating layer is preferably equal to or lessthan 0.9 μm, and more preferably 0.1 to 0.7 μm.

The thickness of each layer and the non-magnetic support of the magneticrecording medium can be measured by known film thickness measurementmethods. For example, a cross section of the magnetic recording mediumin a thickness direction is exposed by known means such as ion beams ora microtome, and then the exposed cross section is observed using ascanning electron microscope. By observing the cross section, athickness of one site in the thickness direction or an arithmetic meanof thicknesses of two or more randomly extracted sites, for example, twosites can be determined as various thicknesses. Furthermore, as thethickness of each layer, a design thickness calculated from themanufacturing condition may be used.

Manufacturing Process

Preparation of composition for forming each layer

The step of preparing a composition for forming the magnetic layer, thenon-magnetic layer, or the back coating layer generally includes atleast a kneading step, a dispersion step, and a mixing step that isperformed if necessary before and after the aforementioned steps. Eachof the aforementioned steps may be divided into two or more stages. Thecomponents used for preparing the composition for forming each layer maybe added at the initial stage or in the middle of any of the abovesteps. As a solvent, it is possible to use one kind of solvent or two ormore kinds of solvents generally used for manufacturing a coating-typemagnetic recording medium. Regarding the solvent, for example, paragraph“0153” in JP2011-216149A can be referred to. Furthermore, each of thecomponents may be added in divided portions in two or more steps. Forexample, the binder may be added in divided . portions in the kneadingstep, the dispersion step, and the mixing step performed afterdispersion to adjust viscosity. In order to manufacture theaforementioned magnetic recording medium, the manufacturing techniquesknown in the related art can be used in various steps. In the kneadingstep, it is preferable to use an instrument having strong kneadingforce, such as an open kneader, a continuous kneader, a pressurizedkneader, or an extruder. For details of the kneading treatment,JP1989-106338A (JP-H01-106338A) and JP1989-79274A (JP-H01-79274A) can bereferred to. As a disperser, known ones can be used. The composition forforming each layer may be filtered by a known method before beingsubjected to a coating step. The filtration can be performed using afilter, for example. As the filter used for the filtration, for example,it is possible to use a filter having a pore size of 0.01 to 3 μm (forexample, a filter made of glass fiber, a filter made of polypropylene,or the like).

Regarding the control of the base material friction, as described above,in an aspect, the magnetic recording medium can be manufactured usingtwo or more kinds of ferromagnetic hexagonal ferrite powders havingdifferent mean particle sizes. That is, the magnetic layer can be formedusing, as ferromagnetic hexagonal ferrite powder, first ferromagnetichexagonal ferrite powder and one or more kinds of ferromagnetichexagonal ferrite powders having a mean particle size larger than thatof the first ferromagnetic hexagonal ferrite powder. As preferredaspects of such a method for forming a magnetic layer, the followingaspects described in (1) to (3) can be exemplified. A combination of twoor more aspects described below is a more preferred aspect of the methodfor forming a magnetic layer. The first ferromagnetic hexagonal ferritepowder refers to one kind of ferromagnetic hexagonal ferrite powderamong two or more ferromagnetic hexagonal ferrite powders to be used andis preferably the aforementioned ferromagnetic hexagonal ferrite powderaccounting for the highest proportion based on mass.

(1) The mean particle size of the first ferromagnetic hexagonal ferritepowder is within a range of 10 to 80 nm.

(2) The difference between the mean particle size of the ferromagnetichexagonal ferrite powder, which has a mean particle size larger thanthat of the first ferromagnetic hexagonal ferrite powder, and the meanparticle size of the first ferromagnetic hexagonal ferrite powder iswithin a range of 10 to 50 nm.

(3) The mixing ratio between the first ferromagnetic hexagonal ferritepowder and the ferromagnetic hexagonal ferrite powder having a meanparticle size larger than that of the first ferromagnetic hexagonalferrite powder based on mass is within a range of 90.0:10.0 to 99.9:0.1(the former:the latter).

Furthermore, in another aspect, a magnetic tape can be manufacturedusing, as non-magnetic powder of a magnetic layer, non-magnetic powderother than the abrasive and the projection-forming agent. That is, themagnetic layer can be formed using the aforementioned anothernon-magnetic powder. As preferred aspects of such a method for forming amagnetic layer, the following aspects described in (4) to (6) can beexemplified. A combination of two or more aspects described below is amore preferred aspect of the method for forming a magnetic layer.

(4) The mean particle size of the aforementioned another non-magneticpowder is larger than the mean particle size of the ferromagnetichexagonal ferrite powder.

(5) The difference between the mean particle size of the ferromagnetichexagonal ferrite powder and the mean particle size of theaforementioned another non-magnetic powder is within a range of 10 to 80nm.

(6) The mixing ratio between the ferromagnetic hexagonal ferrite powderand the aforementioned another non-magnetic powder based on mass iswithin a range of 90.0:10.0 to 99.9:0.1 (the former:the latter).

Regarding the dispersion treatment for the composition for forming amagnetic layer, as described above, it is preferable to inhibit theoccurrence of chipping. In order to inhibit chipping, in the step ofpreparing the composition for forming a magnetic layer, it is preferableto perform the dispersion treatment for the ferromagnetic hexagonalferrite powder in two stages, such that coarse aggregates of theferromagnetic hexagonal ferrite powder are disintegrated in the firststage of the dispersion treatment and then the second stage of thedispersion treatment is performed in which the collision energy appliedto the particles of the ferromagnetic hexagonal ferrite powder due tothe collision with dispersion beads is smaller than in the firstdispersion treatment. According to the dispersion treatment describedabove, it is possible to achieve both of the improvement ofdispersibility of the ferromagnetic hexagonal ferrite powder and theinhibition of occurrence of chipping.

Examples of preferred aspects of the aforementioned two-stage dispersiontreatment include a dispersion treatment including a first stage ofobtaining a dispersion liquid by performing a dispersion treatment onthe ferromagnetic hexagonal ferrite powder, the binder, and the solventin the presence of first dispersion beads, and a second stage ofperforming a dispersion treatment on the dispersion liquid obtained bythe first stage in the presence of second dispersion beads having a beadsize and a density smaller than a bead size and a density of the firstdispersion beads. Hereinafter, the dispersion treatment of theaforementioned preferred aspect will be further described.

In order to improve the dispersibility of the ferromagnetic hexagonalferrite powder, it is preferable that the first and second stagesdescribed above are performed as a dispersion treatment preceding themixing of the ferromagnetic hexagonal ferrite powder with other powdercomponents. For example, in a case where the magnetic layer containingthe abrasive and the aforementioned non-magnetic filler is formed, it ispreferable to perform the aforementioned first and second stages as adispersion treatment for a liquid (magnetic liquid) containing theferromagnetic hexagonal ferrite powder, the binder, the solvent, andadditives optionally added, before the abrasive and the non-magneticfiller are mixed with the liquid.

The bead size of the second dispersion beads is preferably equal to orless than 1/100 and more preferably equal to or less than 1/500 of thebead size of the first dispersion beads. Furthermore, the bead size ofthe second dispersion beads can be, for example, equal to or greaterthan 1/10,000 of the bead size of the first dispersion beads, but is notlimited to this range. For example, the bead size of the seconddispersion beads is preferably within a range of 80 to 1,000 nm. Incontrast, the bead size of the first dispersion beads can be within arange of 0.2 to 1.0 mm, for example.

In the present invention and the present specification, the bead size isa value measured by the same method as used for measuring theaforementioned mean particle size of powder.

The second stage described above is preferably performed under thecondition in which the second dispersion beads are present in an amountequal to or greater than 10 times the amount of the ferromagnetichexagonal ferrite powder, and more preferably performed under thecondition in which the second dispersion beads are present in an amountthat is 10 to 30 times the amount of the ferromagnetic hexagonal ferritepowder, based on mass.

The amount of the first dispersion beads in the first stage ispreferably within the above range.

The second dispersion beads are beads having a density smaller than thatof the first dispersion beads. “Density” is obtained by dividing mass(unit: g) of the dispersion beads by volume (unit: cm³) thereof Thedensity is measured by the Archimedean method. The density of the seconddispersion beads is preferably equal to or lower than 3.7 g/cm³, andmore preferably equal to or lower than 3.5 g/cm³. The density of thesecond dispersion beads may be equal to or higher than 2.0 g/cm³ forexample, and may be lower than 2.0 g/cm³. In view of density, examplesof the second dispersion beads preferably include diamond beads, siliconcarbide beads, silicon nitride beads, and the like. In view of densityand hardness, examples of the second dispersion beads preferably includediamond beads.

The first dispersion beads are preferably dispersion beads having adensity of higher than 3.7 g/cm³, more preferably dispersion beadshaving a density of equal to or higher than 3.8 g/cm³, and even morepreferably dispersion beads having a density of equal to or higher than4.0 g/cm³. The density of the first dispersion beads may be equal to orlower than 7.0 g/cm³ for example, and may be higher than 7.0 g/cm³. Asthe first dispersion beads, zirconia beads, alumina beads, or the likeare preferably used, and zirconia beads are more preferably used.

The dispersion time is not particularly limited and may be set accordingto the type of the disperser used and the like.

Coating Step

The magnetic layer can be formed by directly coating a surface of thenon-magnetic support with the composition for forming a magnetic layeror by performing multilayer coating by sequentially or simultaneouslycoating the support with the composition for forming a non-magneticlayer. The back coating layer can be formed by coating a surface side ofthe non-magnetic support opposite to the surface side which has themagnetic layer (or will be provided with the magnetic layer) with thecomposition for forming a back coating layer. For details of coating forforming each layer, paragraph “0066” in JP2010-231843A can be referredto.

Other Steps

Regarding other various steps for manufacturing the magnetic recordingmedium, paragraphs “0067” to “0070” in JP2010-231843A can be referredto. It is preferable to perform an alignment treatment on the coatinglayer of the composition for forming a magnetic layer while the coatinglayer is staying wet (undried). For the alignment treatment, it ispossible to apply various known techniques including those described inparagraph “0067” in JP2010-231843A without any limitation. As describedabove, from the viewpoint of controlling the XRD intensity ratio, it ispreferable to perform a vertical alignment treatment as the alignmenttreatment. Regarding the alignment treatment, the above description canalso be referred to.

The aforementioned magnetic recording medium according to an aspect ofthe present invention can be a tape-like magnetic recording medium(magnetic tape), for example. Generally, the magnetic tape isdistributed and used in a state of being accommodated in a magnetic tapecartridge. In the magnetic tape, in order to enable head tracking servoto be performed in a drive, a servo pattern can also be formed by aknown method. By mounting the magnetic tape cartridge on a drive(referred to as “magnetic tape device” as well) and running the magnetictape in the drive such that a magnetic head contacts and slides on asurface of the magnetic tape (surface of a magnetic layer), informationis recorded on the magnetic tape and reproduced. In order tocontinuously or intermittently perform repeated reproduction of theinformation recorded on the magnetic tape, the magnetic tape is causedto repeatedly run in the drive. According to an aspect of the presentinvention, it is possible to provide a magnetic tape in which theelectromagnetic conversion characteristics thereof hardly deteriorateeven though the head repeatedly slides on the surface of the magneticlayer while the tape is repeatedly running. Here, the magnetic recordingmedium according to an aspect of the present invention is not limited tothe magnetic tape. The magnetic recording medium according to an aspectof the present invention is suitable as various magnetic recording media(a magnetic tape, a disc-like magnetic recording medium (magnetic disc),and the like) used in a sliding-type magnetic recording and/orreproduction device. The sliding-type device refers to a device in whicha head contacts and slides on a surface of a magnetic layer in a casewhere information is recorded on a magnetic recording medium and/or therecorded information is reproduced. Such a device includes at least amagnetic tape and one or more magnetic heads for recording and/orreproducing information.

In the aforementioned sliding-type device, as the running speed of themagnetic tape is increased, it is possible to shorten the time taken forrecording information and reproducing the recorded information. Therunning speed of the magnetic tape refers to a relative speed of themagnetic tape and the magnetic head. Generally, the running speed is setin a control portion of the device. As the running speed of the magnetictape is increased, the pressure increases which is applied to both thesurface of the magnetic layer and the magnetic head in a case where thesurface of the magnetic layer and the magnetic head come into contactwith each other. As a result, either or both of head scraping andmagnetic layer scraping tend to easily occur. Accordingly, it isconsidered that the higher the running speed, the easier it is for theelectromagnetic conversion characteristics to deteriorate during therepeated sliding. In the field of magnetic recording, the improvement ofrecording density is required. However, as the recording density isincreased, the influence of the signal interference between the adjacentheads becomes stronger, and hence the electromagnetic conversioncharacteristics tend to be more easily deteriorate when the spacing lossis increased due to the repeated sliding. As described so far, as therunning speed and the recording density are increased further, thedeterioration of the electromagnetic conversion characteristics duringthe repeated sliding tends to be more apparent. In contrast, even inthis case, according to the magnetic recording medium of an aspect ofthe present invention, it is possible to inhibit the deterioration ofthe electromagnetic conversion characteristics during the repeatedsliding. The magnetic tape according to an aspect of the presentinvention is suitable for being used in a sliding-type device in whichthe running speed of the magnetic tape is, for example, equal to orhigher than 5 m/sec (for example, 5 to 20 m/sec). In addition, themagnetic tape according to an aspect of the present invention issuitable as a magnetic tape for recording and reproducing information ata line recording density of equal to or higher than 250 kfci, forexample. The unit kfci is the unit of a line recording density (thisunit cannot be expressed in terms of the SI unit system). The linerecording density can be equal to or higher than 250 kfci or equal to orhigher than 300 kfci, for example. Furthermore, the line recordingdensity can be equal to or lower than 800 kfci or higher than 800 kfci,for example. Examples

Hereinafter, the present invention will be described based on examples,but the present invention is not limited to the aspects shown in theexamples. In the following description, unless otherwise specified,“part” and “%” represent “part by mass” and “% by mass” respectively.Furthermore, unless otherwise specified, the steps and the evaluationsdescribed below were performed in an environment with an atmospherictemperature of 23° C.±1° C.

Example 1

The formulations of compositions for forming each layer will be shownbelow.

Formulation of composition for forming magnetic layer

Magnetic Liquid

Plate-like ferromagnetic hexagonal ferrite powder (M-type bariumferrite): 100.0 parts

Two kinds of ferromagnetic hexagonal ferrite powders described belowwere used.

Ferromagnetic hexagonal ferrite powder (1)

-   -   Mean particle size and formulation ratio: see Table 1

Ferromagnetic hexagonal ferrite powder (2)

-   -   Mean particle size and formulation ratio: see Table 1

Oleic acid: 2.0 parts

Vinyl chloride copolymer (MR-104 manufactured by ZEON CORPORATION): 10.0parts

SO₃Na group-containing polyurethane resin: 4.0 parts

(weight-average molecular weight: 70,000, SO₃Na group: 0.07 meq/g)

Amine-based polymer (DISPERBYK-102 manufactured by BYK-Chemie GmbH): 6.0parts

Methyl ethyl ketone: 150.0 parts

Cyclohexanone: 150.0 parts

Abrasive liquid

α-Alumina: 6.0 parts

(BET specific surface area: 19 m²/g, Mohs hardness: 9)

SO₃Na group-containing polyurethane resin: 0.6 parts

(weight-average molecular weight: 70,000, SO₃Na group: 0.1 meq/g)

2,3-Dihydroxynaphthalene: 0.6 parts

Cyclohexanone: 23.0 parts

Projection-forming agent liquid

Colloidal silica: 2.0 parts

-   -   (mean particle size: 80 nm)

Methyl ethyl ketone: 8.0 parts

Lubricant and curing agent liquid

Stearic acid: 3.0 parts

Amide stearate: 0.3 parts

Butyl stearate: 6.0 parts

Methyl ethyl ketone: 110.0 parts

Cyclohexanone: 110.0 parts

Polyisocyanate (CORONATE (registered trademark) L manufactured by TosohCorporation): 3.0 parts

Formulation of composition for forming non-magnetic layer

Non-magnetic inorganic powder a iron oxide: 100.0 parts

-   -   (mean particle size: 10 nm, BET specific surface area: 75 m²/g)

Carbon black: 25.0 parts

-   -   (mean particle size: 20 nm)

SO₃Na group-containing polyurethane resin: 18.0 parts

-   -   (weight-average molecular weight: 70,000, content of SO₃Na        group: 0.2 meq/g)

Stearic acid: 1.0 part

Cyclohexanone: 300.0 parts

Methyl ethyl ketone: 300.0 parts

Formulation of composition for forming back coating layer

Non-magnetic inorganic powder a iron oxide: 80.0 parts

-   -   (mean particle size: 0.15 μm, BET specific surface area: 52        m²/g)

Carbon black: 20.0 parts

-   -   (mean particle size: 20 nm)

Vinyl chloride copolymer: 13.0 parts

Sulfonate group-containing polyurethane resin: 6.0 parts

Phenyl phosphonate: 3.0 parts

Cyclohexanone: 155.0 parts

Methyl ethyl ketone: 155.0 parts

Stearic acid: 3.0 parts

Butyl stearate: 3.0 parts

Polyisocyanate: 5.0 parts

Cyclohexanone: 200.0 parts

Preparation of Composition for Forming Magnetic Layer

The composition for forming a magnetic layer was prepared by thefollowing method.

The aforementioned various components of a magnetic liquid weredispersed for 24 hours by a batch-type vertical sand mill by usingzirconia beads (first dispersion beads, density: 6.0 g/cm³) having abead size of 0.5 mm (first stage) and then filtered using a filterhaving a pore size of 0.5 μm, thereby preparing a dispersion liquid A.The amount of the used zirconia beads was 10 times the total mass of theferromagnetic hexagonal barium ferrite powders (1) and (2) based onmass.

Then, the dispersion liquid A was dispersed for the time shown in Table1 by a batch-type vertical sand mill by using diamond beads (seconddispersion beads, density: 3.5 g/cm³) having a bead size shown in Table1 (second stage), and the diamond beads were separated using acentrifuge, thereby preparing a dispersion liquid (dispersion liquid B).The following magnetic liquid is the dispersion liquid B obtained inthis way.

The aforementioned various components of an abrasive liquid were mixedtogether and put into a horizontal beads mill disperser together withzirconia beads having a bead size of 0.3 mm, and the volume thereof wasadjusted such that bead volume/(volume of abrasive liquid) +bead volumeequaled 80%. The mixture was subjected to a dispersion treatment byusing the beads mill for 120 minutes, and the liquid formed after thetreatment was taken out and subjected to ultrasonic dispersion andfiltration treatment by using a flow-type ultrasonic dispersion andfiltration device. In this way, an abrasive liquid was prepared.

The prepared magnetic liquid and abrasive liquid as well as theaforementioned projection-forming agent liquid, the lubricant, and thecuring agent liquid described above were introduced into a dissolverstirrer, stirred for 30 minutes at a circumferential speed of 10 m/sec,and then treated in 3 passes with a flow-type ultrasonic disperser at aflow rate of 7.5 kg/min. Thereafter, the resultant was filtered througha filter having a pore size of 1 μm, thereby preparing a composition forforming a magnetic layer.

Preparation of Composition for Forming Non-Magnetic Layer

The aforementioned various components of a composition for forming anon-magnetic layer were dispersed by a batch-type vertical sand mill for24 hours by using zirconia beads having a bead size of 0.1 mm and thenfiltered using a filter having a pore size of 0.5 μm, thereby preparinga composition for forming a non-magnetic layer.

Preparation of Composition for Forming Back Coating Layer

Among the aforementioned various components of a composition for forminga back coating layer, the components except for the lubricant (stearicacid and butyl stearate), polyisocyanate, and 200.0 parts ofcyclohexanone were kneaded and diluted using an open kneader and thensubjected to a dispersion treatment in 12 passes by a horizontal beadsmill disperser by using zirconia beads having a bead size of 1 mm bysetting a bead filling rate to be 80% by volume, a circumferential speedof the rotor tip to be 10 m/sec, and a retention time per pass to be 2minutes. Then, other components described above were added thereto,followed by stirring with a dissolver. The obtained dispersion liquidwas filtered using a filter having a pore size of 1μm, thereby preparinga composition for forming a back coating layer.

Preparation of Magnetic Tape

A surface of a support made of a polyethylene naphthalate having athickness of 5.0 μm was coated with the composition for forming anon-magnetic layer prepared as above such that a thickness of 100 nm wasobtained after drying, and then the composition was dried, therebyforming a non-magnetic layer. A surface of the formed non-magnetic layerwas coated with the composition for forming a magnetic layer prepared asabove such that a thickness of 70 nm was obtained after drying, therebyforming a coating layer. While the coating layer of the composition forforming a magnetic layer is staying wet (undried), a vertical alignmenttreatment was performed on the coating layer such that a magnetic fieldhaving the intensity shown in Table 1 was applied in a directionperpendicular to a surface of the coating layer. Then, the coating layerwas dried.

Thereafter, a surface of the support opposite to the surface on whichthe non-magnetic layer and the magnetic layer were formed was coatedwith the composition for forming a back coating layer prepared as abovesuch that a thickness of 0.4 μm was obtained after drying, and then thecomposition was dried. By using a calender constituted solely with metalrolls, a calender treatment (surface smoothing treatment) was performedon the obtained tape at a speed of 100 m/min, a line pressure of 300kg/cm (294 kN/m), and a calender roll surface temperature of 90° C.Subsequently, the tape was subjected to a heat treatment for 36 hours inan environment with an atmospheric temperature of 70° C. After the heattreatment, the tape was slit in a width of 1/2 inches (0.0127 meters),and a servo pattern was Ruined on the magnetic layer by using acommercially available servowriter.

In this way, a magnetic tape of Example 1 was obtained.

Evaluation of deterioration of electromagnetic conversioncharacteristics (Signal-to-Noise-Ratio; SNR)

The electromagnetic conversion characteristics of the magnetic tape ofExample 1 were measured using a ½-inch (0.0127 meters) reel tester, towhich a head was fixed, by the following method.

The running speed of the magnetic tape (relative speed of magnetichead/magnetic tape) was set to be the value shown in Table 1. AMetal-In-Gap (MIG) head (gap length: 0.15 track width: 1.0 μm) was usedas a recording head, and as a recording current, a recording currentoptimal for each magnetic tape was set. As a reproducing head, aGiant-Magnetoresistive (GMR) head having an element thickness of 15 nm,a shield gap of 0.1 and a lead width of 0.5 μm was used. Signals wererecorded at a line recording density shown in Table 1, and thereproduced signals were measured using a spectrum analyzer manufacturedby ShibaSoku Co., Ltd. A ratio between an output value of carriersignals and integrated noise in the entire bandwidth of the spectrum wastaken as SNR. For measuring SNR, the signals of a portion of themagnetic tape, in which signals were sufficiently stabilized afterrunning, were used.

Under the above conditions, each magnetic tape was caused to performreciprocating running in 5,000 passes at 1,000 m/l pass in anenvironment with a temperature of 40° C. and a relative humidity of 80%,and then SNR was measured. Then, a difference between SNR of the 1^(st)pass and SNR of the 5,000^(th) pass (SNR of the 5,000^(th)) pass—SNR ofthe 1^(st) pass) was calculated.

The recording and reproduction described above were performed by causingthe head to slide on a surface of the magnetic layer of the magnetictape.

Examples 2 to 17

Magnetic tapes were prepared in the same manner as in Example 1 exceptthat various items shown in Table 1 were changed as shown in Table 1,and the deterioration of the electromagnetic conversion characteristics(SNR) of the prepared magnetic tapes was evaluated.

In Table 1, in the example for which “N/A” is described in the column ofDispersion beads and the column of Time, the composition for forming amagnetic layer was prepared without performing the second stage in thedispersion treatment for the magnetic liquid.

In Table 1, in the example for which “N/A” is described in the column ofMagnetic field intensity for vertical alignment treatment, the magneticlayer was formed without performing the alignment treatment.

The formulation ratio of the ferromagnetic hexagonal ferrite powderdescribed in Table 1 is a mass-based content ratio of each ferromagnetichexagonal ferrite powder with respect to a total of 100.0 parts by massof the ferromagnetic hexagonal ferrite powders. The mean particle sizeof the ferromagnetic hexagonal ferrite powder shown in Table 1 is avalue obtained by collecting the powder in a required amount from thepowder lot used for preparing the magnetic tape and measuring the meanparticle size thereof by the method described above. The ferromagnetichexagonal ferrite powder having undergone the measurement of the meanparticle size was used for preparing the magnetic liquid for preparingthe magnetic tape.

A portion of each of the prepared magnetic tapes was used for theevaluation of the deterioration of electromagnetic conversioncharacteristics (SNR), and the other portion thereof was used forphysical property evaluation described below.

Evaluation of Physical Properties of Magnetic Tape

(1) XRD Intensity Ratio

From each of the magnetic tapes of Examples 1 to 17, tape samples werecut.

By using a thin film X-ray diffractometer (SmartLab manufactured byRigaku Corporation), X-rays were caused to enter a surface of themagnetic layer of the cut tape sample, and In-Plane XRD was performed bythe method described above.

From the X-ray diffraction spectrum obtained by In-Plane XRD, a peakintensity Int (114) of a diffraction peak of (114) plane and a peakintensity Int (110) of a diffraction peak of (110) plane of thehexagonal ferrite crystal structure were determined, and the XRDintensity ratio (Int (110)/Int (114)) was calculated.

(2) Squareness Ratio in Vertical Direction

For each of the magnetic tapes of Examples 1 to 17, by using a vibratingsample fluxmeter (manufactured by TOEI INDUSTRY, CO., LTD.), asquareness ratio in a vertical direction was determined by the methoddescribed above.

(3) Base Material Friction

First, by using a laser marker, marks were made on a measurementsurface, and an atomic force microscope (AFM) image of a portionseparated from the marks by a certain distance (about 100 μm) wasmeasured. The measurement was performed by setting a visual field areato be 7 μm×7 μm. At this time, in order to make it easy to capture aScanning Electron Microscope (SEM) image of the same site as will bedescribed later, the cantilever was changed to a hard one(monocrystalline silicon), and marks were made using AFM. From the AFMimage measured in this way, all the projections having a height of equalto or greater than 15 nm from the reference plane were extracted. Then,the site where the presence of a projection was not confirmed wasspecified as a base material portion, and the base material friction wasmeasured using the TI-950 TRIBOINDENTER manufactured by Hysitron by themethod described above.

Furthermore, an SEM image of the same site as that measured using AFMwas measured to obtain a component map, and it was confirmed that theextracted projections having a height of equal to or greater than 15 nmfrom the reference plane are projections formed of alumina or colloidalsilica. In each example, none of alumina and colloidal silica wereconfirmed in the base material portion in the component map obtainedusing SEM. Herein, the component analysis was performed using SEM.However, the component analysis is not limited to SEM, and can beperformed by known methods such as Energy Dispersive X-ray Spectrometry(EDS) and Auger Electron Spectroscopy (AES).

The results obtained as above are shown in Table 1.

TABLE 1 Dispersion treatment for magnetic liquid Second stage Dispersionbeads Formulation amount (mass of beads Ferromagnetic Ferromagnetic withrespect to hexagonal hexagonal total mass of ferrite powder (1) ferritepowder (2) ferromagnetic Mean Mean hexagonal ferrite particleFormulation particle Formulation Bead powders (1) size ratio size ratioType size and (2)) Time Example 1 22 nm  99.0% 60 nm 1.0% Diamond 500 nm10 times greater 1 h Example 2 22 nm  99.0% 60 nm 1.0% Diamond 500 nm 10times greater 1 h Example 3 22 nm  99.0% 60 nm 1.0% Diamond 500 nm 10times greater 1 h Example 4 22 nm  99.0% 60 nm 1.0% Diamond 500 nm 10times greater 1 h Example 5 22 nm  99.0% 60 nm 1.0% Diamond 500 nm 20times greater 1 h Example 6 22 nm  99.0% 60 nm 1.0% Diamond 500 nm 10times greater 1 h Example 7 22 nm  98.8% 60 nm 1.2% Diamond 500 nm 10times greater 1 h Example 8 22 nm  98.5% 60 nm 1.5% Diamond 500 nm 10times greater 1 h Example 9 22 nm 100.0% — — N/A N/A N/A N/A Example 1022 nm 100.0% — — N/A N/A N/A N/A Example 11 22 nm 100.0% — — N/A N/A N/AN/A Example 12 22 nm 100.0% — — Diamond 500 nm 10 times greater 1 hExample 13 22 nm  99.0% 60 nm 1.0% N/A N/A N/A N/A Example 14 22 nm 99.0% 60 nm 1.0% N/A N/A N/A N/A Example 15 22 nm  99.0% 60 nm 1.0% N/AN/A N/A N/A Example 16 22 nm  99.0% 60 nm 1.0% Diamond 500 nm 10 timesgreater 1 h Example 17 22 nm  99.0% 60 nm 1.0% Diamond 500 nm 10 timesgreater 1 h Magnetic field XRD intensity for intensity SquarenessRunning vertical Base ratio ratio in speed of Line alignment materialInt (110)/ vertical magnetic recording SNR treatment friction Int (114)direction tape density deterioration Example 1 0.15 T 0.30 0.5 0.69 6m/s 270 kfci −0.5 dB Example 2 0.20 T 0.30 1.5 0.75 6 m/s 270 kfci −0.6dB Example 3 0.30 T 0.30 2.4 0.81 6 m/s 270 kfci −0.5 dB Example 4 0.50T 0.30 4.0 0.85 6 m/s 270 kfci −0.5 dB Example 5 0.15 T 0.30 0.7 0.83 6m/s 270 kfci −0.4 dB Example 6 0.30 T 0.30 2.4 0.81 8 m/s 300 kfci −0.8dB Example 7 0.30 T 0.25 2.4 0.81 8 m/s 300 kfci −0.5 dB Example 8 0.30T 0.22 2.4 0.81 8 m/s 300 kfci −0.3 dB Example 9 N/A 0.45 0.2 0.55 4 m/s200 kfci −1.0 dB Example 10 N/A 0.45 0.2 0.55 6 m/s 270 kfci −3.0 dBExample 11 N/A 0.45 0.2 0.55 8 m/s 300 kfci −4.5 dB Example 12 0.15 T0.45 0.5 0.70 6 m/s 270 kfci −2.5 dB Example 13 N/A 0.30 0.3 0.56 6 m/s270 kfci −2.5 dB Example 14 0.15 T 0.30 3.8 0.62 6 m/s 270 kfci −2.0 dBExample 15 0.30 T 0.30 5.0 0.76 6 m/s 270 kfci −2.4 dB Example 16 1.00 T0.30 6.2 0.88 6 m/s 270 kfci −2.1 dB Example 17 N/A 0.30 0.3 0.65 6 m/s270 kfci −2.5 dB

From the results shown in Table 1, it was confirmed that in Examples 1to 8, in which each of the XRD intensity ratio, the squareness ratio ina vertical direction, and the base material friction of the magnetictape is within the range described above, the electromagnetic conversioncharacteristics hardly deteriorate even though reproduction is repeatedby causing the head to slide on the surface of the magnetic layer,unlike in Examples 9 to 17.

In Examples 9 to 11, magnetic tapes of the same physical properties wereused, but the magnetic tapes had different running speeds and differentline recording densities. Through the comparison between Examples 9 to11, it is possible to confirm that as the running speed or the linerecording density of the magnetic tape is increased, the deteriorationof the electromagnetic conversion characteristics during the repeatedsliding becomes more apparent. In Examples 1 to 8, the deterioration ofthe electromagnetic conversion characteristics during the repeatedsliding could be inhibited.

One aspect of the present invention can be useful in the technical fieldof magnetic recording media for data storage such as data backup tapes.

What is claimed is:
 1. A magnetic recording medium comprising: anon-magnetic support; and a magnetic layer which is provided on thesupport and contains ferromagnetic powder and a binder, wherein theferromagnetic powder is ferromagnetic hexagonal ferrite powder, themagnetic layer contains an abrasive, an intensity ratio (Int (110)/Int(114)) of a peak intensity Int (110) of a diffraction peak of (110)plane of a crystal structure of the hexagonal ferrite, determined byperforming X-ray diffraction analysis on the magnetic layer by using anIn-Plane method, to a peak intensity Int (114) of a diffraction peak of(114) plane of the crystal structure is equal to or higher than 0.5 andequal to or lower than 4.0, a squareness ratio of the magnetic recordingmedium in a vertical direction is equal to or higher than 0.65 and equalto or lower than 1.00, and a coefficient of friction measured in a basematerial portion within a surface of the magnetic layer is equal to orlower than 0.30.
 2. The magnetic recording medium according to claim 1,wherein the squareness ratio in a vertical direction is equal to orhigher than 0.65 and equal to or lower than 0.90.
 3. The magneticrecording medium according to claim 1, wherein the coefficient offriction measured in a base material portion within a surface of themagnetic layer is equal to or higher than 0.15 and equal to or lowerthan 0.30.
 4. The magnetic recording medium according to claim 2,wherein the coefficient of friction measured in a base material portionwithin a surface of the magnetic layer is equal to or higher than 0.15and equal to or lower than 0.30.
 5. The magnetic recording mediumaccording to claim 1, further comprising: a non-magnetic layercontaining non-magnetic powder and a binder between the non-magneticsupport and the magnetic layer.
 6. The magnetic recording mediumaccording to claim 1, further comprising: a back coating layercontaining non-magnetic powder and a binder on a surface, which isopposite to a surface provided with the magnetic layer, of thenon-magnetic support.
 7. The magnetic recording medium according toclaim 1, which is a magnetic tape.